1. Field of the Invention
This invention relates to thermoplastic polyester-urethane molding compositions having certain advantageous molding and physical properties for use in sealing mechanisms where low compression set and good physical properties are required.
2. Description of the Prior Art
Epsilon-caprolactone is known to polymerize with a polyhydric alcohol or a glycol-type material to form a hydroxy terminated polyester which, when mixed along with a diol chain extender and a diisocyanate of the organic type, can be used to form relatively long chain polyester-polyurethane polymers. Such polymers are presently known in the industry to be useful in many applications including injection and compression molding.
Exemplary of art using caprolactones is U.S. Pat. No. 3,658,756 which discloses the use of a caprolactone hydrolyzate or alcoholizate which is reacted with a glycol to form a polyester material. Such polyester material is then reacted with an organic diisocyanate and a chain extender such as a glycol material to form a polyester-urethane material.
It is also known to utilize excess ratios of diisocyanate to isocyanate-reactable hydroxyl groups in the polyester-polyurethanes in order to form allophanate cross-linking bonds. More specifically, the excess isocyanates can react through the carbon atom of such isocyanate to the nitrogen atom in the urethane grouping in the polyester-polyurethane thereby forming the allophanate cross-links.
It has been found that such allophanate linkages break down at relatively high temperatures and reform upon cooling forming a material which is thermosetting in character. At the higher temperature the allophanate linkage can break or be severed allowing the material to possess more of the thermoplastic properties which enhance molding operations.
However, I have found that, in addition to the above use of the epsilon-caprolactone with chain extenders and diisocyanates and the use of excess diisocyanate to produce allophanate linkages, unexpected improvement properties can result when there is incorporated in the reactant mixture an organic siloxane polymer. Such polymer provides and also increases the physical properties of the molded product. In a specific instance such addition will increase tensile strength, elongation percentage and a 100 and 300% modulus elasticity properties.